In order to ensure the best possible performance of a transmitter or data packet signal generator in a modern communication system, testing would typically be required to ensure that the transmitter output power setting is close to or at an optimal performance. Performance usually includes many parameters like output power, phase noise, IQ mismatch, spurious transmissions, etc. Often all modulation quality parameters (e.g. phase noise, IQ mismatch, and the like) are combined into a single value representing all of the transmitter impairments (including noise impairment), the single value known as the error vector magnitude (EVM). The EVM represents how far the measured transmitter constellation is from the ideal constellation.
EVM can be measured in dB or %, where EVMdb=20*LOG10 (evm%/100). Different standards may define different requirements on how EVM is measured, but the basis of the measurement is usually the same. Looking at the transmitter specifications, some requirements relate to regulatory specifications, and other requirements relate more directly to transmitter performance.
The regulatory specifications include maximum transmitted power (usually specified at peak and/or at average) as well as spurious transmission (spurious power generated away from the desired frequency spectrum). These parameters are typically tested during regulatory compliance by making measurements using a power meter and a spectrum analyzer. Sufficient margins may be chosen to guard against reaching the regulatory limits such that limited testing may only be needed during production testing. Testing for compliance to regulatory limits may be very time consuming.
Transmitter performance is usually defined by two parameters, the output power and the modulation accuracy (measured by EVM). Typically, the highest output power with the best possible EVM is desired. However, EVM and output power are somewhat correlated such that when the transmitter or data packet signal generator starts to experience compression from increased power, the EVM normally increases (becomes worse). A trade off is usually needed to reach a close to optimal performance.
In older, less modern systems, higher transmit power could usually be applied to yield better performance until the spectral requirements limit would be reached, the spectral requirements being the limiting factor in increasing the power. However, with modern communication systems, the EVM begins to degrade much sooner with the increase of power, and EVM may become a limiting factor sooner than spectral requirements as power is increased.
Typically, EVM is good for low power, and as the power increases the EVM gradually becomes worse. If the EVM is too good, the system performance is dominated by noise at the receiver (e.g. the signal to noise ratio (SNR) is too close to the theoretical SNR limit). As more power is added, better system performance is obtained until the EVM becomes so bad that the system will simply stop functioning with the smallest amount of interference. Accordingly, an optimal EVM level exists where overall system performance is acceptable for the increased power output, and further increase in power output decreases the performance due to the worsening EVM value.
Traditionally, EVM has been difficult to measure, as well as time consuming, compared to traditional power measurements. Typically, the output power of a device has been calibrated to a value that ensures the EVM meeting the EVM requirement. However, in this case, output power is chosen low enough so that all devices can be guaranteed to pass the EVM specification or target EVM. As a result, calibration of transmitters has been non-optimal.
Ideally, a combined EVM and power calibration should be performed, where EVM is adjusted to the desired limit or target EVM by increasing the output power until the EVM target limit is met or the maximum power is reached as allowed by regulatory requirements. In this way, adjusting for optimal performance for all devices would be done instead of ensuring the output power of all devices is constant.
However, EVM is a time consuming measurement. As EVM is a number including all impairments to the transmitted signal, EVM also includes noise, and so a single repeated measurement of a signal will not yield a constant EVM. The EVM will exhibit some statistical variation. Thus, multiple measurements at a given output power level may be needed to establish an averaged or true EVM (EVM averaging). Also, calculating EVM is a computationally intensive operation compared to traditional power measurement. As a consequence, increased test time may be needed to calibrate each transmitter due to the multiple measurements needed for each true EVM measurement point, and the number of EVM measurement points needed to establish calibration at the target EVM level.
In view of the above, improvements are needed to determine a time efficient manner for calibration of a transmitter or data packet signal generator to a more optimal performance based on EVM and output power level.